Tires may be prepared with treads composed of diene-based rubber compositions which contain reinforcing filler comprised of a combination of rubber reinforcing carbon black and precipitated silica. The precipitated silica may be used together with a silica coupling agent to couple the precipitated silica to diene-based elastomers contained in the rubber composition.
Here it is desired to provide a rubber composition for a vehicular tire tread having a relatively low hysteresis to thereby promote relatively low internal heat generation during tire service with an accompanying low internal temperature rise and beneficially low rolling resistance for a tire with tread of such rubber composition.
To promote such property for such rubber composition for a tire tread, it is desired for its reinforcing filler to contain a relatively low level (low content) of rubber reinforcing carbon black.
However, providing a significantly low level of rubber reinforcing carbon black in the rubber composition also promotes a significantly high electrical resistivity (low electrical conductivity) for the rubber composition to thereby resist transmission of electrical energy through the rubber composition for dissemination of the electrical energy from the tread to the ground.
Therefore, a challenge is presented for providing a tire tread of a rubber composition containing reinforcing filler comprised of a relatively low level of rubber reinforcing carbon black yet which also promotes significant electrical conductivity.
For such challenge, it is proposed to evaluate providing relatively incompatible dual rubber phases, such as a synthetic cis 1,4-polybutadiene rubber based rubber phase and a natural cis 1,4-polisoprene rubber based rubber phase. They are considered to be relatively incompatible with each other in the sense of having solubility parameters which are significantly different from each other.
It is proposed for the natural rubber based rubber composition to be a continuous rubber phase containing a dispersed cis 1,4-polybutadiene based rubber phase. The natural rubber phase is contemplated as being the continuous rubber phase in a sense of forming a network within the dual phase system which has a continuous path for conduction of an electrical charge.
It is proposed for the filler reinforcement of the natural rubber based continuous phase to be primarily rubber reinforcing carbon black and therefore exist as a relatively electrically conductive rubber phase. It is proposed for the filler reinforcement for the cis 1,4-polybutadiene based rubber phase to be primarily precipitated silica with little or no rubber reinforcing carbon black and therefore exist as a relatively electrically resistive rubber phase.
It is also proposed to resist the migration of the aforesaid carbon black and precipitated silica rubber reinforcing fillers between the aforesaid rubber phases to thereby promote maintenance of their respective electrical conductivity and electrical resistivity properties.
In particular, it is proposed to promote resistance of significant migration of the rubber reinforcing carbon black from the dispersed natural rubber based rubber phase into the cis 1,4-polybutadiene based rubber phase by relying on inherent affinity of the rubber reinforcing carbon black for the natural rubber based rubber phase in a sense of the ability of natural rubber or cis 1,4-polyisoprene to produce bound rubber with carbon black as a result of its ability to create high shear mixing and produce a polymer break down structure that binds it to carbon black surface. This binding with carbon black will help resist the migration of the carbon black to the cis 1,4-polybutadiene phase.
In particular, it is proposed to promote resistance to significant migration of the precipitated silica from the dispersed cis 1,4-polybutadiene based rubber based rubber phase into the continuous natural rubber based rubber phase by providing a silica coupler to couple the precipitated silica to the cis 1,4-polybutadiene rubber. A coupling of the precipitated silica is promoted by a combination of reaction of coupler with hydroxyl groups (e.g. silanol groups) of the precipitated silica and interaction with the carbon-to-carbon bonds of the diene-based elastomer(s) of the cis 1,4-polybutadiene based rubber phase.
The term “phr” where used herein, and according to conventional practice, refers to “parts of a respective material per 100 parts by weight of rubber, or elastomer”.
The terms “rubber” and “elastomer” where used herein, are to be used interchangeably, unless otherwise indicated. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art. The terms “vulcanize” and “cure” where used herein are used interchangeably unless otherwise indicated.